Switching mode power supplies (SMPS) are used in a variety of applications including notebook adapters, liquid crystal display (LCD) television adapters, battery chargers, consumer appliances such as Digital Video Discs (DVD) and video cassette recorders, wireless access points, Internet Protocol (IP) phones, etc. Most of the high efficiency switching mode power supplies in use today use Synchronous Rectification (SR) to achieve the desired efficiency for an SMPS power stage. Switching mode power supplies typically include an SR controller for controlling a Synchronous Rectification Metal Oxide Semiconductor Field Effect Transistor (SR MOSFET) switch that bypasses a standard rectifier. FIG. 1 is a circuit schematic of a portion of a secondary side 12 of an SR system in an SMPS 10. For the sake of clarity the connections on primary side 14 of SMPS 10 have been omitted. What is shown in FIG. 1 is a secondary side synchronous rectification controller 16, an SR MOSFET 18, a Schottky diode 20, a secondary winding 22, a filtering capacitor 25, and a load 27 mounted to a printed circuit board 28. By way of example, load 27 comprises a resistor. Controller 16 may be, for example, a synchronous rectification controller having part number NCP4302 and manufactured by Semiconductor Components Industries, LLC and serves to set a threshold voltage for turning off SR MOSFET 18. Controller 16 has an output 30 connected to a gate terminal of SR MOSFET 18, a current sensing input 32 connected to the drain terminal of SR MOSFET 18, and an input 34 connected to ground which serves as a current return pin. Controller 16 includes a comparator 36 having an output connected to an internal logic circuit 41, which has an output coupled for driving the gate of SR MOSFET 18. Examples of internal logic circuit 41 are known to those skilled in the art. For example, the data sheet for part number NCP4302 manufactured by Semiconductor Components Industries, LLC illustrates suitable circuitry for logic circuit 41. Comparator 36 also has an input connected to input 32 and an input coupled to a voltage source 38 that provides a threshold reference voltage VTHR. SR MOSFET 18 has a body diode 44 between its source and drain and parasitic drain and source inductances represented by parasitic drain inductor 40 and a parasitic source inductor 42. Parasitic inductors 40 and 42 are the result of electrically conductive elements such as leads and bond wires that are within the packaging material of SR MOSFET 18. Schottky diode 20 is connected across the drain and source terminals of SR MOSFET 18.
In operation, controller 16 determines the drain-to-source voltage of SR MOSFET 18 and uses this voltage in combination with the threshold reference voltage VTHR to set the turn-off current level of the current flowing through SR MOSFET 18. Typically, threshold reference voltage VTHR is selected to be near zero so that the turn-off current is small or substantially equal to zero. A secondary current (ISEC) flows from filtering capacitor 25 and load 27 through SR MOSFET 18 towards secondary winding 22, creating a voltage drop across parasitic drain and source inductors 40 and 42, respectively, that increases the turn-off current. The increased turn-off current is caused by an inaccurate determination of the voltage drop across the channel of the SR MOSFET, i.e., by an inaccurate determination of the value of the channel voltage resulting from the product of secondary current ISEC and resistance Rds—ON. The inaccuracy results from the voltages developed across parasitic elements associated with printed circuit board 28 and the voltage developed across parasitic inductors 40 and 42 associated with SR MOSFET 18. When SMPS 10 operates in a discontinuous conduction mode (DCM) or, alternatively, when SMPS 10 operates as a series resonant converter such as, for example, an inductor-inductor capacitor (LLC) series resonant converter at a frequency that is below its series resonant frequency, it is desirable to detect when secondary current ISEC reaches a zero value.
A commonly used technique to compensate for parasitic voltage drops across the parasitic passive circuit elements associated with printed circuit board 28 involves measuring the drain-to-source voltage using a Kelvin sensing probe. However, this technique does not account for voltage drops across parasitic inductors 40 and 42 associated with SR MOSFET 18. Thus, the determination of the channel voltage (which equals ISEC*Rds—ON) is inaccurate because it is derived from a voltage measurement that includes the voltages across parasitic inductors 40 and 42.
A drawback with the circuit shown in FIG. 1 is that secondary current ISEC is still flowing when the drain-to-source voltage is zero. Thus, SR MOSFET 18 still carries a significant secondary current ISEC. Because SR MOSFET 18 is turned off, channel conduction and the efficiency the SR system within SMPS 10 are decreased. The effect of parasitic inductors 40 and 42 becomes more serious in high frequency applications where the change of current with respect to time increases and the SR MOSFET Rds—ON value decreases.
In addition, parasitic inductors 40 and 42 create a phase shift between the drain current and the drain-to-source voltage which results in an increased turn-off current for SR MOSFET 18 that changes with load current. When threshold reference voltage VTHR has a negative voltage, the turn-off current of SR MOSFET 18 is even higher.
Accordingly, it would be advantageous to have a method and structure for compensating for parasitic components within a transistor. It would be of further advantage for the method and structure to be cost efficient to implement.